1. Field of the Invention
This invention relates to an arrangement for electrically heating a semiconductor rod which is simultaneously growing due to a deposition thereon of materials from a gas phase. More specifically, the invention relates to such a heating arrangement which is comprised into a heating current circuit together with a heating current source which supplies alternating currents U(t) having a period T and an electronic switch which can be controlled by a time-wise variable auxiliary voltage V(t), which heating current circuit conducts the heating current I(t) supplied from the heating current source. The heating current I(t) is effective by way of a second circuit which is inductively coupled to the heating current circuit and has rectifier characteristics and to a smoothing four terminal circuit whereby the difference .DELTA. = G - G.sub.O of the direct voltage G which is received at the output of the smoothing circuit serves with its desired value G.sub.O to control a generator for the auxiliary voltage in such a way that the auxiliary voltage is activated earlier for larger values of the difference .DELTA..
2. Description of the Prior Art
During the production of a highly pure crystalline semiconductor material according to the above mentioned method the depositing process is generally directed at several rods of small diameter which thereafter, during the running of the process gradually grow due to the deposition thereon of the semiconductor material. In the case of arrangements heretofore used for heating the rods, the heating current is generally taken from a voltage source which is connected to the public mains. Since this voltage source is almost equal to a source of constant voltage, and since the process requires a source of constant current, the heating arrangements incorporate current limiting components which regulate the heating current to a constant value due to the negative resistance characteristic of the semiconductor material and, for example, consist of phase controlled thyristors.
Since the heat dissipation to the surroundings for a prescribed rod temperature, which has to be maintained, depends on the surface of the semiconductor rod, the heating current which is regulated to a constant value due to the growth of the rod must increase during the deposition process in proportion to the rod surface.
The electronic switch which is controlled by way of a regulator in such a way that the heating current is maintained at a constant level offers itself as the best current limiting component, whereby the regulating magnitude of the actual current value is the heating current itself.
The heating current increases, as already shown during the course of the deposition process, since the electrical energy which is transformed in the semiconductor rod for maintaining the desired temperature equal the heat dissipation to the surroundings, the heat dissipation itself, with other conditions of the surface remaining unchanged, being proportional to the diameter of the semiconductor rod.
It has been proven that it is necessary and also advisable, in the case of arrangements which were heretofore employed, to arrange the current control in such a way that as large a current control range as possible, for example 1:100 can be achieved. However, special measures are required since the methods which are generally used for the current control lead to an undesirable amount of imperfections and the quality of the deposited material does not correspond to the requirements for the semiconductor rods, with respect to purity and homogeneity of the semiconductor rod, and in particular in the case of rational operation of the arrangement the quality of the deposited material does not correspond to the requirements with respect to the desired thickness of the rod. This large range of control has, however, high requirements with respect to the control components, particularly the amplifier, so that extraordinary requirements are made either with respect to the components as far as quality and the admissible imperfection limits are concerned, or on the other hand the amplification of the current control has to be changed during the running of the process.